Carried by Elenwen at the Thalmor Embassy, Ancano at the College of Winterhold, and Ondolemar in Understone Keep in Markarth. Either npc can be pickpocketed for the blade, or you can complete the College of Winterhold questline, [>>>Quest Spoiler Alert<<<] during which you kill Ancano, and loot the blade from him.
Each of the three Thalmor members has their own version of the blade, which carries either a fire (Elenwen), frost (Ancano), or shock enchantment (Ondolemar). The display will take only one.
Requires Royal Armory.
"Based on ancient Aldmerian design, these swords are the pinnacle of Altmer craftsmanship. They are also extremely expensive to produce, reserving them almost exclusively to the most important within the Thalmor ranks."